To achieve an improvement in reliability of vehicle bodies and a reduction in weight of the vehicle bodies for the purpose of an improvement in fuel consumption, strengthening of steel sheets has been pursued in recent years. The use of high-strength steel sheets allows vehicle bodies to have stiffness comparable to that obtained using conventional steel even when the steel sheets are reduced in thickness and weight. However, some problems have been pointed out. One of them is a reduction in the strength of welded portions.
As shown in FIG. 1, in resistance spot welding, a sheet set 3 including at least two overlapping steel sheets (in this case, two steel sheets including a lower steel sheet 1 and an upper steel sheet 2) is held between a vertical pair of electrodes (a lower electrode 4 and an upper electrode 5), and an electric current is applied under pressure to melt a contact region between the steel sheets 1 and 2. A nugget 9 of the required size is thereby formed, and a weld joint is obtained.
The quality of the joint obtained in the manner described above is evaluated by, for example, the diameter of the nugget, tensile shear strength (the strength when a tensile test is performed in a shear direction of the joint), cross-tension strength (the strength when a tensile test is performed in a separation direction of the joint), or fatigue strength. In particular, as the strength of a steel sheet increases, the amount of C in the steel sheet tends to increase. It is known that, in a high-strength steel sheet containing a large amount of C, its cross-tension strength is low.
In terms of the welding method, to ensure the cross-tension strength when the high-strength steel sheet is used, it is contemplated to increase the diameter of the nugget. Generally, to increase the size of the nugget, an electric current must be increased. In this case, the possibility of the occurrence of expulsion becomes high. If expulsion occurs, the nugget is rather reduced in size, and this causes a reduction in the strength of the joint.
In particular, the surfaces of steel sheets for automobiles are subjected to galvanization treatment with zinc as a main component for the purpose of rust prevention. It is known that, when steel sheets having such galvanized layers are used for automobiles and subjected to resistance spot welding to assemble the automobiles, expulsion is likely to occur, and it is therefore difficult to ensure large nuggets.
A conventional technique disclosed in Patent Literature 1 is a method for forming nuggets in three stacked steel sheets. In this method, after a first welding step is performed, second and subsequent welding steps are performed such that the supply of electric current and the suspension of the supply are repeated in a pulsated manner. This allows nuggets with sufficient diameters to be formed even when the sheet set including three sheets is composed of a thin sheet, a thick sheet, and a thick sheet.
It is stated in Patent Literature 2 that, when steel sheets having on their surfaces alloyed aluminum coating layers containing Fe at an atomic percentage of from 50% to 80% inclusive are welded, a nugget can be formed stably by specifying, according to the thickness of the sheets, the period of time during which the current is held constant after upslope energization.
It is stated in Patent Literature 3 that, with zinc or zinc alloy coated steel sheets, a nugget having a certain size can be ensured by limiting the ratio of the period of preliminary energization to the period of the formation of the nugget.
It is stated in Patent Literature 4 that, with zinc or zinc alloy coated steel sheets, a nugget having a certain size can be ensured by, after preliminary energization, repeating cooling and energization using a current value higher than the current value for the preliminary energization.